Abstract
在1993至 1997年間，本研究以機動、快速計讀的FHT-751中子偵檢器藉著車載、艦載、機載、人員攜帶進行全方位量測台灣本島、離島及外島地表及空中的宇宙中子背景輻射，更在台灣海域到南海海域完成了地磁度0至16度北的海面宇宙中子量測。本研究除了建立太平島水樣、沙樣、指標植物及海產的基本輻射背景資料外，並量取宇宙中子及環境加馬輻射背景劑量率。此一攜帶方便、靈敏度高的 BF3中子偵檢器系統，經雜波校正、干擾測試、中子源水平校準、亂向入射計算及宇宙中子回應計算後，解算其絕對效率。其效率轉換參數約在1,000 nSvh-1cps-1，視中子能譜而略有變化。 本研究發現，台灣地區宇宙中子強度隨著地表高度增加而昇高 ; 500公尺以下的海岸平原並沒有明顯的變化，其數據跳動咸信與雲雨、水氣、溼度變化有關。台灣地區位於全球宇宙射線較弱地帶，地表量測遠低於高緯度的UNSCEAR 均值。本研究亦執行高空飛行台北?屏東?東沙空域的飛航宇宙中子量測，對台灣空域飛勤組員受到宇宙中子暴露的輻射危險度進行評估 ; 在0?16度磁北的大片海域內，亦即是南海至台海海域上，宇宙中子強度隨著大氣壓力及雲層厚薄而變動。而太平島上的樣品偵測值較台灣本島稍低，加馬能譜計測量得全為低活度天然背景核種，室外的直接加馬劑量率比台灣地區略低。本研究亦討論宇宙中子在空-陸，空-船-海-陸，空-船-海等介面的中子減速、反射、吸收能力各有不同，也導致量測結果相異。 此外，在1996年7-8月問，強烈颱風賀伯襲台，颱風眼正好通過新竹地區中子偵檢器位置，本研究趁便全程監測颱風期間宇宙中子劇烈的變動。宇宙中子強度顯然深受大氣壓影響，其強度與雲量的厚薄成正比，這也是全球首次全程追蹤颱風內宇宙中子場劇烈變動的研究。A portable, rug-proved neutron counter was used to monitorthe cosmic neutron radiation around Taiwan in this work. TheFHT-751 neutron countingsystem, including the BF3 counter,notebook computer, and hand-carried power supply, was calibratedof its noise level, checked with intense gamma interference, andusing a 252Cf neutron source to resolve the counting efficiency.By use of the MCNP transport code, the absolute efficiency fordetecting cosmic neutrons with energy spread to 12 orders ofmagnitude from all direction was calculated around l000nSvh-1cps-1, depending upon the cosmic neutron energy spectra. TheFHT-75l neutron counting system was carried by van to monitorthe surface cosmic neutron dose on Taiwan, it was also hand-carried to the mountains in central Taiwan, up to the top ofMount Yushan (3952m). It was found that the cosmic neutron doserate increases as the ground altitude increases, the neutronintensity at Mount Yushan is 22 times of that measured atcoastal plain. The fluctuation of cosmic neutron intensity dataat ground surface below 500 m is due primarily to the change ofweather conditions such as humidity and cloud coverage. Airborne monitoring of in-flight cosmic neutron exposure was alsoconducted in this work. The detector was carried by militarytransport plane and utility helicopter and flew around the airspace over Taiwan Air Defense Identification Zone. The in-flightdata at cruising altitude is substantially higher than any datameasured on the ground. The detector was also carried by navalvessels to survey the sea-level cosmic neutron intensity in boththe Formosa Strait and the South China Sea where the verticalcut-off of geomagnetic rigidity is the strongest in the world.The detector was also brought to the Itu Aba Islet of theSpratly group in the South China Sea, where the geomagnetic zerodegree is passed right over it, to monitor the world's lowestcosmic neutron background radiation on this tiny coral islet.The influence of air-sea, air-land, and air-ship interfaces forcosmic neutron intensity is diagnosed, resulting in highestreading for shipboard survey, followed by docking measurementand inland monitoring, respectively. Apparently, the high energycosmic neutron is moderated, reflected, and absorbed more insoil than in sea water. More interestingly, a rare event wasencountered in this work that drastic variation of ground cosmicneutron intensity was recorded during typhoon strike. Wereported the first time the onset of neutron dose in typhoon;this is due to the low air pressure, thick spiral cloud, heavyprecipitation and strong gale associated with the strikingtyphoon. The cosmic neutrons can easily penetrate the typhooneye to which the air pressure within is the lowest, resulting infour times higher than the normal reading of cosmic neutronintensity on the ground.